Diseases of the glomerulus, the filtering unit of the kidney, account for over 60% of all cases of end-stage renal disease, which represents a major health problem worldwide. Among them are Goodpasture's (GP) disease, Alport syndrome and diabetic nephropathy that damage the glomerular basement membrane (GBM) through pathogenic mechanisms involving collagen IV scaffolds. This proposal focuses on the collagen IV scaffolds with the overall goal to gain a deeper insight into fundamental mechanisms of molecular recognition, at the atomic level, that underlie normal assembly, function and dysfunction of the scaffolds. Our recent discoveries reveal a GP autoantigen complex that plays a central role in GBM function and in the pathogenesis in GP and Alport diseases. This complex now serves as the cornerstone for four Specific Aims that target an understanding of the chemistry and pathology of each component. The knowledge gained will provide a framework for development of new forms of therapy. Aim 1: To determine the molecular structure of GP epitopes of a345 collagen IV network and role of GP antibody in pathogenesis. We identified four homologous hotspot regions within ?3NC1 and ?5NC1 monomers that are key for GP antibody binding and have cloned first human GP monoclonal antibody. We hypothesize that the full epitopes are formed upon the change in antigen conformation and that epitope-specific GP mAbs will induce renal failure. Aim 2: To determine the impact of PXDN dysregulation on a345 collagen IV networks. We discovered that PXDN catalyzes a121 sulfilimine crosslinks, requires Br, makes HOBr, and anti-PXDN antibodies occur in GP sera. We hypothesize that PXDN crosslinks ?345 collagen IV through an enzyme-substrate complex, and that perturbation of this complex creates oxidative protein modifications on collagen IV. Aim 3: To determine whether GP epitope formation requires phosphorylation by GPBP. GPBP has emerged as a nonconventional kinase that binds and phosphorylates the GP autoantigen and influences the assembly of collagen IV networks. We hypothesize that GPBP phosphorylation alters the presentation of Ea and Eb epitopes and promotes autoantibody binding to ?3NC1. Aim 4: To determine the NC1-mediated mechanism for assembly of the collagen IV ?345 network. We discovered that NC1 domains of collagen IV function as assembly recognition modules. We hypothesize that NC1 domains mediate the assembly of protomers and networks by distinct mechanisms, which are disrupted in disease.